Keep a torch handy if you try the mig nozzles.
You may need to apply a bunch of heat to get them flowing with the long exposed (to air) surface.
As to the sizes, keep reading on castboolits, it is in there.
The nicest looking home made nozzles I had seen on there were bolts that were drilled fairly large up the inside if the shank, an a small hole off at an angle that joined it. Worked well for the guy and is pretty close to a ringer for the commercial nozzles, except you can buy a hundred bolts and the drill bits for the price of a couple $6 or $9 nozzles.
Cheers
Trev